ELF Executable and Linkable Format
Standard binary format for Linux, BSD, Solaris, mobile OSes. Replaced a.out + COFF in Unix. Supports executable files, shared libraries (.so), and object files (.o).
Sections (.text = code, .data = initialized data, .bss = zeroed, .rodata = read-only, .symtab = symbols). Program headers define segments the loader maps into memory. Tools: readelf -a shows everything, objdump -d disassembles, nm lists symbols, ldd shows shared library deps. RELRO + PIE + stack canaries are modern security features in ELF. macOS uses Mach-O instead, Windows uses PE/COFF.